1. Field of the Invention
The present invention relates to a CVD apparatus, and particularly, to a CVD apparatus suitable for depositing films on large flat-panel substrates.
2. Description of the Related Art
Conventionally, known production methods for large liquid crystal displays include a method which uses high-temperature polysilicon TFTs (thin film transistors) and a method which uses low-temperature polysilicon TFTs. The production method employing the high-temperature polysilicon TFTs uses a quartz substrate that withstands high temperatures of 1,000° C. or more to produce high-quality oxide films. On the other hand, the method employing the low-temperature polysilicon TFTs uses a glass substrate as is generally used for TFTs, requiring film deposition to be performed in a lower-temperature environment (for example, 400° C.). The production method for liquid crystal displays using the low temperature polysilicon TFT has the advantage of not requiring use of a special substrate. This method, therefore, has come into practical use in recent years, enjoying a continuous increase in the production.
In the production of liquid crystal displays that employs the low-temperature polysilicon TFTs, plasma-enhanced CVD is adopted in the low-temperature deposition of silicon oxide films suitable as gate insulation films. The deposition of silicon oxide films with the plasma-enhanced CVD uses a gas such as silane or tetraethoxysilane (TEOS) as a typical reactive gas.
In the deposition of silicon oxide films with the plasma-enhanced CVD by use of silane as the reactive gas, conventional plasma-enhanced CVD apparatuses perform the film deposition in the following manner. Gases, such as a reactive gas and oxygen, are fed in a front-side space of a substrate, a gas mixture of the reactive gas and the oxygen is used to produce a plasma, and the substrate is then exposed to the plasma so as to form the silicon oxide film on the surface of the substrate. In this way, the conventional plasma-enhanced CVD apparatuses are configured to allow the reactive gas to feed directly into the plasma produced in the plasma-enhanced CVD apparatuses. When using a conventional plasma-enhanced CVD apparatus, problems may arise when high-energy ions are implanted on film deposition surfaces from the plasma existing in the front-surface space of the substrate. Such action causes damage to the silicon oxide films, and film characteristics are reduced. In addition, since the reactive gas is fed directly in the plasma, the reactive gas and oxygen react vigorously, producing dust particles. This causes a problem in that the yield is reduced.
To overcome the above problems, as an example of conventional cases, there is a proposal for a plasma-processing apparatus that uses a plasma-isolating method. In the plasma-isolating method, a configuration is used so that short-lived charged particles isolated from a plasma-producing region of a plasma apparatus disappear, and a substrate is placed in a region where radicals which live relatively long predominantly exist, and concurrently, a reactive gas is fed close to the region in which the substrate is placed. The radicals produced in the plasma region diffuse toward the region in which the substrate has been placed and are fed into a front-surface space of the substrate. In the above plasma-processing apparatus of the plasma-isolating method, advantages are provided in that vigorous reaction between the plasma and the reactive gas is suppressed, dust particles are reduced, and in addition, implantation of ions into the substrate is restricted.
Also, conventionally, a plasma-enhanced CVD apparatus is proposed in Japanese Unexamined Patent Publication No. 6-260434 (Japanese Patent No. 2,601,127). The proposed plasma-enhanced CVD apparatus has a parallel-flat-plate type electrode structure. In this configuration, an intermediate electrode is arranged between a radio-frequency electrode and a substrate-holder electrode, partitioning a space between the radio-frequency electrode and the substrate-holder electrode. Concurrently, RF power is supplied only between the radio-frequency electrode and the substrates-holder electrode. In this manner, a plasma discharge is generated only between the radio-frequency electrode and the substrate-holder electrode, and excited active species and ions which have been generated by the plasma discharge are fed into a front-surface space of a substrate through through-holes formed in the intermediate electrode. The radio-frequency electrode is of a conventional shower-head type, and a plasma-producing gas is fed into a plasma-generating space through a plurality of holes formed in a diffusion plate. The reactive gas is fed into the front-surface space of the substrate through gas-feeding spaces and gas-discharging openings which are formed in the intermediate electrode. This plasma-enhanced CVD apparatus has a configuration in which the space between the radio-frequency electrode and the substrate-holder electrode is partitioned by means of the intermediate electrode, and only the space between the radio-frequency electrode and the intermediate electrode is formed as the plasma-generating space. As a result, the plasma-producing region is isolated from a position where the substrate is placed. This plasma-enhanced CVD apparatus can be considered to be a modification of the apparatus of the plasma-isolating method which has the parallel-flat-plate type electrode structure.
Also proposed conventionally is a plasma-enhanced CVD apparatus in accordance with Japanese Unexamined Patent Publication No. 5-21393. The proposed plasma-enhanced CVD apparatus has a plasma-producing chamber and a substrate-processing chamber inside a vacuum vessel that forms the CVD apparatus of the parallel-flat-plate type, and has a mesh plate at a border section between the chambers.
Furthermore, conventionally, a plasma-processing apparatus is proposed according to Japanese Unexamined Patent Publication No. 8-167596. This plasma-processing apparatus has a vacuum vessel in which a metal mesh plate and a support member therefor is arranged to separate the inside space into a plasma-producing chamber and a plasma-processing chamber. According to this plasma-processing apparatus, the diameter of a plurality of openings formed in the mesh plate is determined to be twice as large as the Debye length of a plasma produced in the plasma producing chamber. This shields charged particles in the plasma, and excited atomic species which are electrically neutral are emitted on a processed object.
In the plasma-processing apparatus of the plasma-isolating method, as described above, the plasma producing region and the substrate-placing region are isolated by a communication space. Also, the radicals produced in a region isolated from the substrate travel through the communication space, and using effects of diffusion occurring therein, the radicals are fed onto the surface of the substrate. Therefore, problems arise in that the deposition speed is reduced, and the distribution of the radicals is not suitable in the vicinity of the surface of the substrate. Particularly, the fact that the distribution of the radicals is not suitable has given rise to a problem in that the capacity does not meet requirements for large-surface substrates used for large liquid crystal displays.
According to the plasma-enhanced CVD apparatus disclosed in Japanese Unexamined Patent Publication No. 6-260434 (Japanese Patent No. 2,601,127), advantages are highlighted as follows. The reactive gas is not supplied to the plasma-generating space between the radio-frequency electrode and the intermediate electrode. Therefore, no chemical reactions occur around the radio-frequency electrode, films are not accumulated on the radio-frequency electrode, and furthermore, dust particles are not formed. According to careful investigation, however, no particular considerations are exerted on the dimensions of the through-holes formed on the intermediate electrode. In this case, a possibility remains in that the reactive gas will diffuse back into the plasma-generating space. Therefore, there is a probability that the reactive gas will enter into an upper side of the intermediate electrode, causing chemical reactions around the radio-frequency electrode.
In the plasma-enhanced CVD apparatus disclosed in Japanese Unexamined Patent Publication No. 5-21393 also, the determined dimensions of the through-holes formed in the mesh plate are such that the reactive gas may diffuse back into the plasma-generating space, causing the same problem as in the above case.
The plasma-processing apparatus disclosed in Japanese Unexamined Patent Publication No. 8-167596 has a configuration in which movement of charged particles from the plasma-generating space to the plasma-processing chamber is blocked. However, there is no description regarding a configuration that will avoid a possibility that the reactive gas which has been fed into the plasma-processing chamber so as not to contact the plasma will diffuse back into the plasma-generating chamber through the plurality of openings formed in the mesh plate. Therefore, there is a possibility that the reactive gas will enter into the plasma-generating chamber through the mesh plate, causing chemical reactions with the plasma.